In my opinion, you need to buy at least one more piece of equipment to get consistently good macro photos: either (1) a tripod or (2) a flash. It is so difficult to create high-quality macro photos without either of these things that I don't think it's even worth trying. However, the good news is that you can do either one for relatively cheap when compared to the cost of your body and lens.
You can get a decent enough tripod and head for under $200. You would use these if you want to do natural light macro photography. For examples of what's possible without stacking, check out this natural-light set of fly photos from Heath McDonald. He uses a slightly longer lens (150mm, I think...), but the results you can get with the 100mm will be similar. When you have a tripod on your side, you can create great images out of single exposures, so there is no need for stacking unless you decide you want extreme depth of field.
http://www.flickr.com …72157630071897374/detail/
For a flash, you could try the strategy that Nikola Rahmé is using in the photo below. He cut out a piece of plastic and attached it to the lens for diffusion, and the flash is just some variation of a Canon 270 exii, which is also under $200. He's one of the world's best macro photographers, so if it's good enough for him, it's good enough for the rest of us.
http://www.flickr.com/photos/eurythyrea/4907251110/
Of course, there are plenty of other ways you can handle the whole flash situation. I have seen many cheap alternatives that seem to work great. In the end, though, I highly recommend either a flash or tripod solution. You can certainly take adequate photos with just the lens and camera body, but the number of situations in which this combination will produce awe-inspiring work is limited.
